variables not shown in templates

[FloatBird]

hello,

I don't understand this. I made a plugin for general use like this:

hook name: global_start (i checked also other hooks but nothing working)
plugin code:

$myvar = "vb4 plugin not showing variables";

and in 'navbar' template I added this:

{vb:var myvar}

but unfortunately, myvar not shown in 'navbar' ..
it's work if I do a new hook and add to the template, or create new template for just this single variable? help please.